Whole body protein synthesis: studies with different amino acids in the rat

These studies were undertaken to determine to what extent constant infusion measurements and plasma sampling could provide sensible answers for rates of whole body protein turnover and also which amino acid would be the most representative probe of whole body protein turnover. Whole body protein synthesis rates were estimated in 70-g rats with L-[U-14C]threonine, L-[U-14C]lysine, L-[U-14C]tyrosine, L-[U-14C]phenylalanine, and L-[1-14C]leucine by either simultaneous tracer infusion of four amino acids or by injections of large quantities of 14C-labeled amino acids. In the infusion experiment, indirect estimates of whole body protein turnover based on free amino acid specific radioactivity and stochastic modeling were compared with direct measurement of the incorporation of the tracer into proteins. These two methods of analysis provided similar results for each amino acid, although in each case fractional synthesis rates were lower (by between 26 and 63%) when calculations were based on plasma rather than tissue specific radioactivity. With the flooding-dose method, whole body fractional protein synthesis rates were 41.4, 25.6, 31.1, and 31.4% with threonine, lysine, phenylalanine, and leucine, respectively. These values were similar to those obtained by the continuous infusion method using tissue specific radioactivity for threonine and lysine. For leucine, however, the flooding-dose method provided an intermediate value between the two estimates derived either from the plasma or the tissue specific radioactivity in the infusion method.(ABSTRACT TRUNCATED AT 250 WORDS)